A typical device for manufacturing an electrolytic copper foil comprises a metal cathode drum and an insoluble metal anode (anode), the metal cathode drum being rotatable and having a mirror polished surface. The insoluble metal anode is arranged at approximately the lower half of the metal cathode drum and surrounds the metal cathode drum. A copper foil is continuously manufactured with the device by flowing a copper electrolytic solution between the cathode drum and the anode, applying a direct current between these to allow copper to be electrodeposited on the cathode drum, and detaching an electrodeposited copper foil from the cathode drum when a predetermined thickness is obtained. The side that the electrolytic copper foil contacts with the surface of the titanium drum is referred to as “shiny side (S side),” and the back side of the electrolytic copper foil is referred to as “matte side (M side).”
The copper foil is often used as a negative current collector for lithium-ion secondary batteries and for printed wiring boards. Lithium-ion secondary batteries include a positive electrode, a negative electrode, and an electrolyte. The negative electrode typically includes carbon particles applied as a negative electrode active material layer (active carbon material) to the surface of a negative current collector made from the copper foil.
There are two types of active carbon materials that are applied to the negative current collector (the copper foil): an aqueous slurry and a solvent-based (non-aqueous slurry). The aqueous slurry uses different binders and solvents than the solvent-based slurry. The aqueous slurry usually includes a styrene-butadiene rubber (SBR), a binder, and water as the solvent. The solvent-based slurry usually includes polyvinylidene fluoride (PVDF), a binder, and a solvent such as 1-methyl-2-pyrrolidone (NMP).
Although aqueous slurries account for about 90% of the current market, solvent slurries are actually better suited for lithium-ion batteries because water is harmful to the lithium-ion battery. Solvent-based slurries are more commonly used in high-end products (for example, high C-rate charging and discharging). The Li-ion batteries for EV and power tools need to have a high C-rate charging and discharging.
When using a solvent-based slurry it is necessary for the surface of the copper foil to have an affinity to the solvent slurry to achieve good adhesion between the copper foil and the carbon active material. The slurry is applied to the copper foil and then dried with heat for up to about 10 minutes to remove the solvent (at about 100-160° C.). For solvent-based slurries that use NMP, a higher drying temperature is needed because the solvent has a higher boiling point than water. At higher temperatures, however, copper foil is more susceptible to oxidation, which leads to discoloration. Furthermore, lithium-ion batteries are often subjected to high temperatures during repeated use while charging and/or discharging (temperatures of 55° C., 5C charge and 5C discharge). When high C-rate charging and discharging is employed and temperatures rise, the anode active material layer can break down causing the batteries to fail.
The capacity of a rechargeable battery is commonly referred to as the “C-rate.” The capacity of a rechargeable battery is often rated at 1C, meaning that a 1,000 mAh battery should provide a current of 1,000 mA for an hour. The same battery discharging at 0.5C would provide 500 mA for two hours, and at 2C, the 1,000 mAh battery would deliver 2,000 mA for 30 minutes. 1C is also known as a one-hour discharge; a 0.5C is a two-hour discharge, and a 2C is a half-hour discharge. When discharging a battery with a battery analyzer capable of applying different C rates, a higher C rate will produce a lower capacity reading and vice versa. By discharging the 1,000 mAh battery at the faster 2C, or 2,000 mA, the battery should ideally deliver the full capacity in 30 minutes. The sum should be the same as with a slower discharge since the identical amount of energy is being dispensed, only over a shorter time. In reality, however, internal resistance turns some of the energy into heat and lowers the resulting capacity to about 95 percent or less. Discharging the same battery at 0.5C, or 500 mA over two hours, will likely increase the capacity to above 100 percent.